1. Field of the Invention
The present invention relates generally to receiving signals from satellite navigational systems, and more particularly, to the simultaneous reception of signals from satellites of multiple Global Navigational Satellite Systems' (GNSS) constellations.
2. Description of the Related Art
Satellite navigational systems provide positional and timing information to earth-bound receivers. Each system has its own constellation of satellites orbiting the Earth, and, in order to calculate its position, a receiver on Earth uses the satellites “in view” (i.e., in the sky above) from that system's constellation. Global Navigational Satellite Systems (GNSS) is often used as the generic term for such systems, even though such navigational satellite systems include regional and augmented systems—i.e., systems that are not truly “global.” The term “GNSS,” as used herein, covers any type of navigational satellite system, global or not, unless expressly indicated otherwise.
The number of GNSS systems, both planned and presently operational, is growing. The widely-known, widely-used, and truly global Global Positioning System (GPS) of the United States has been joined by one other global system, Russia's GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS), and is presently being joined by Europe's Galileo and China's BeiDou (also known, in its second generation, as COMPASS) systems—each of which has, or will have, its own constellation of satellites orbiting the globe. Regional systems (those that are not global, but intended to cover only a certain region of the globe) include Japan's Quasi-Zenith Satellite System (QZSS) and the Indian Regional Navigational Satellite System (IRNSS) currently being developed. Augmented systems are normally regional as well, and “augment” existing GNSS systems with, e.g., messages from ground-based stations and/or additional navigational aids. These include the Wide Area Augmentation System (WAAS), European Geostationary Navigation Overlay Service (EGNOS), Multi-functional Satellite Augmentation System (MSAS), and GPS Aided Geo Augmented Navigation (GAGAN). Regional GNSS systems, such as QZSS, can also operate as augmented systems.
The four operating or soon-to-be-operating truly “global” GNSSs, i.e., GPS, GLONASS, Galileo, and BeiDou, are providing, and/or will be providing, an unprecedented number of satellites overhead by which a GNSS receiver can calculate its position using the “open service” channels transmitted by each GNSS system. GPS transmits an open service L1 signal, and had 32 working satellites in its constellation as of December 2012, which constitute 24 operational satellites, with 4 satellites in 6 different orbital planes, guaranteeing there are at least 6 overhead at any time at any spot on Earth. GLONASS transmits an open service L1 signal, and, as of July 2013, has 29 satellites in its constellation, with 23 being operational. Galileo transmits an open service E1 signal, and has 30 satellites planned, spread out in three orbital planes, and intends to guarantee there will be 6-8 overhead at any time at “most locations.” BeiDou-2, also known as COMPASS, transmits an open service B1 signal, and will have 35 satellites in its constellation.
Thus, in the near future, a minimum of 30 and a maximum of more than 50 satellites may be available overhead for a GNSS receiver at any time—if the GNSS receiver is capable of receiving signals from all four GNSS systems. However, the various GNSS systems use different signal structures, and most of them use different frequencies, making the simultaneous reception of signals from satellites in all 4 GNSS constellations difficult without excessive power consumption and/or receiver apparatus complexity.
Thus, a solution is needed for a GNSS receiver to simultaneously receive satellite signals from all 4 GNSS constellations, without excessive power needs and/or undue device complexity.